Doing a walk thru?

[Ctskip]

I have a 1983 31 ft Allmand I would like to do a walk-thru transom and first I'd do a split rear stay. Then cut down through the combing/transom into the propane locker to the floor which is the same height as the cockpit deck , Take the center of the helmsmen seat out for the walk thru. There is a brace for the rudder post 8 inches below the deck from one side to the other and I would reinforce it. How about flex? Any thoughts? thank you.
Keep it up,
Ctskip

 

[sailingdog]

By cutting the transom that way, you're going to seriously compromise the strength of the transom of the boat. You're basically cutting the back end of the box that forms the cockpit off. You're also weakening the area that supports the rudder and the loads the rudder takes, as well as the aft chain plates for the backstay.

I would highly recommend hiring a marine architect or engineer to oversee such a drastic modification of the stern.

 

[alexco38]

How about mount a swim platform on the stern?

 

[Ctskip]

There's a 1 inch shelf about 8 inches below deck specifically designed to support the rudder post. The post goes through this shelf and the shelf is adhered (fiberglassed to) to the transom on the inside of the hull from one side to the other side. It's almost like two decks. One to walk on and the other just for the support of the rudder post. I'm also not taking the entire back side off. Just about 18" from the top of the combing or so and maybe 15 inches out of the middle Just the width of the ladder. I'd still be leaving about 30 inches or so to the water line. Just to the floor of the propane locker. So not only will the floor be reinforced, side to side, the shelf for the rudder post be reinforced also. I am also thinking about a sugar scoop instead of a add on platform.
I thought Glenn was a marine architect and or engineer? I guess because I can see it and can envision what it'll look like, I see no problem. Thats why i added the picture. I always tend to over do what ever it is I do anyway. I just thought I'd ask a pro.

Keep it up,
Ctskip

 

[sailingdog]

Yes, but the transom and the center helmsman seat both act as box-beams to help connect the two sides of the cockpit and the hull together, giving it support at the stern. Removing those will adversely affect the rigidity of the hull more likely than not. Imagine a shoebox container filled with marbles... now imagine cutting a big flap out of one of the short ends of the shoebox. Chances are likely that it will flex considerably more now under the weight of supporting the marbles... that's what you'd effectively be doing to the hull of your boat.

There's a 1 inch shelf about 8 inches below deck specifically designed to support the rudder post. The post goes through this shelf and the shelf is adhered (fiberglassed to) to the transom on the inside of the hull from one side to the other side. It's almost like two decks. One to walk on and the other just for the support of the rudder post. I'm also not taking the entire back side off. Just about 18" from the top of the combing or so and maybe 15 inches out of the middle Just the width of the ladder. I'd still be leaving about 30 inches or so to the water line. Just to the floor of the propane locker. So not only will the floor be reinforced, side to side, the shelf for the rudder post be reinforced also. I am also thinking about a sugar scoop instead of a add on platform.
I thought Glenn was a marine architect and or engineer? I guess because I can see it and can envision what it'll look like, I see no problem. Thats why i added the picture. I always tend to over do what ever it is I do anyway. I just thought I'd ask a pro.

Keep it up,
Ctskip

 

[Ctskip]

Now take a beam to support the sides, albeit lower than the coming and reinforce this twice, one at deck level and the second about 8 inches lower (rudder post support shelf). You make it sound like I'm taking a "big flap" out of the transom. Just a small section, the width of the ladder and down to the deck. Maybe 18 wide by 24 deep. Not big at all.
How do they do it when there is no transom at all? My major concern is the amount of flex and inward pull from the rear stays pulling in and forces while underway. I feel that with the two reinforced supports it would be fine. But I'm no engineer. Thats why I was trying to ask one. I would imagine that the inward force on a open tapered transom would be considerably less than what it would be in the middle of the vessel. I just don't know what forces exist and how to compensate for them.
I know people tell me to buy a boat with a open transom. I like my boat just fine. Better than fine. I just want certain amenities, that's all. Like a wind charger and some dingy davits off the stern for solar panels. Not to too much to ask for is it?

Keep it up,
Ctskip

 

[ghen]

Ctskip,

The modification you desire can be done but it will require more analysis than can be provided in this forum. It is a serious undertaking particularily since the rudder is involved. I suspect you may need the services of a N.A.. We fear giving cursory advice when structural modifications are made because of potential disasterous results. We don't know how much glass exist or bracing without detailed measurements. Then calculations are made to determine the requirements.

 

[Ctskip]

Just as I expected. I can understand the apprehension because of potential liability factors. Thank you for your response.
I don't want to sound ignorant , but what is a N.A.?

The shelf for the rudder post bearing 8 inches below the deck, will be reinforced, I'm not touching it otherwise. I see it as insurance for any possible flexing that could happen. The cockpit floor will also be reinforced underneath from wall to wall. I'll start with the split rear stay.
Thanks everyone for the input.

Being one to fabricate and do what some see as the impossible,(which takes a little longer) I'll try this one of these days and I'll let y'all know how it turns out. I have noticed that lately it has been getting harder and harder to find a place where one can work on their own boat. Lawyers, arg!!

Keep it up,
Ctskip

 

[sailingdog]

I recently added a bridge deck to my boat. However, I didn't consult a naval architect as I was adding strength to the existing boat, rather than removing something. Generally, it is far riskier to remove something that is already there than it is to add something to the existing structure. My boat is definitely stiffer than before the bridgedeck was added. The companionway, which is a big opening in the cabin top, is now a third smaller than it was previously, and the cockpit has been reinforced by a 1' x 1' box beam that is the bridgedeck.

You say that you're not cutting a large opening, but you're planning on cutting a two-foot tall opening into the transom and removing three square feet of transom as well as the helmsman seat, which are both essentially box beams across the stern end of the boat. The loss of the support they provide is for both inward loads, like those created by the chainplates for the backstay, and for outward loads cause by the backstay pulling up on the hull.

The transoms of boats with a walk through transom are generally designed and engineered to support those loads. The transom of your boat was not.

 

[ghen]

N.A. stands for Naval Architect. I depend upon an aft bulkhead to support the lateral loads. Lots of stuff is going on back there with the rudder and backstay (on boats with a backstay). I did a lot of mods on my own boats like that before I formally learned. An N.A. loathes giving out hopefully helpful information but it not be because he (me) doesn't have all the facts. Thanks for your understanding.

 

[Franklin]

Skip:

I do believe it is possible to make your boat more open, however, I do think it's a lot more work then you are thinking about. I'll use my H376 open transom boat as an example: The hull has twice as much wood in the stern then it does midship at the chainplates. This wood gives it the neccessary rigidness it needs. It also has a lot of curves on the sides that give it sideways strength. There is a decent amount of glasswork inside the cockpit to add strength. Even with all that, when I'm offshore, it sure does creek a lot along that stern bulkhead which is part of the additional support which I don't think you have.

Making an open transom sterdy is very difficult just like it is building a Cat with two hulls and keeping those hulls together in bad weather offshore. Reminds me of the 100' cat that split on it's madden voyage getting ready for a race.

I know you are thinking it's only the top 18", but that part needs something to keep it from flexing too, so you would need to re-inforce the hull, the cockpit walls, put in a bulkhead down below and also build up the floor of the cockpit. It's not a task I would want to take on and I can guarantee you that the value of your boat would drop to almost zero as everybody would be shaky about it if you tried to sale it later on.